1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a liquid crystal display (LCD) device for obtaining high aperture ratio and wide viewing angle.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device includes lower and upper substrates facing each other with a predetermined interval therebetween, and a liquid crystal layer between the lower and upper substrates. The liquid crystal layer is driven by an electric field generated between the lower and upper substrates, thereby displaying an image.
Among the LCD devices, a Twisted Nematic (TN) mode liquid crystal display (LCD) device has been most generally used. In TN mode, longitudinal directions of liquid crystal molecules between the lower and upper substrates are parallel with the lower and upper substrates, and the liquid crystal molecules are spirally twisted with a predetermined pitch, so that the longitudinal directions of the liquid crystal molecules are aligned to change continuously.
The TN mode LCD device has characteristics of varying the transmittance of light at each gray level in accordance with a corresponding viewing angle. Specifically, the transmittance of light is distributed symmetrically in right and left directions of the TN mode LCD device, but asymmetrically in lower and upper directions, whereby gray inversion is generated. In order to overcome such a problem, a method is proposed to compensate for the variation of light transmittance in accordance with a corresponding viewing angle by providing domains with differentiating an alignment directions of a liquid crystal layer in a pixel region.
An example of a multi-domain technology dividing a pixel region into a plurality of domains is Two Domain Twisted Nematic (TDTN) mode, in which one pixel region is divided into two domains by a boundary at the center portion of the pixel region, and liquid crystal molecules are symmetrically aligned in the two domains in accordance with the boundary. The TDTN mode has a problem in that manufacturing process steps are complicated due to repetitive photolithography process steps. In this respect, a method for forming an auxiliary electrode and an electric field inducing window is applied to the TDTN mode, which has been usually applied to a vertical alignment (VA) mode LCD device. Herein, the method for forming the auxiliary electrode and the electric field inducing window is applied to a TN mode LCD device.
A related art LCD device will be explained with reference to the accompanying drawings. FIG. 1A is a plan view illustrating a related art LCD device, and FIG. 1B is a cross-sectional view illustrating a related art LCD device taken along line I-I′ of FIG. 1A when voltage is applied.
Referring to FIG. 1A and FIG. 1B, the related art LCD device includes lower and upper substrates 10 and 20 facing each other, and liquid crystal molecules 31 between the lower and upper substrates 10 and 20. A plurality of gate and data lines 11 and 12 are formed on the lower substrate 10 and each other to define a plurality of pixel regions. A pixel electrode 15 is formed in each pixel region. An auxiliary electrode 13 is formed on the same layer as the data line 12, and the auxiliary electrode 13 is formed at the periphery of the pixel electrode 15. Then, a gate insulating layer (not shown) is formed between the gate and data lines 11 and 12, and a passivation layer 14 is formed between the data line 12 and the pixel electrode 15. Although not shown, a thin film transistor including a gate electrode, a semiconductor layer, and source/drain electrodes is formed at each crossing point of the gate and data lines 11 and 12.
An upper substrate 20 includes a black matrix layer 21 that prevents light from leaking in regions other than the pixel region, a color filter layer 22 for displaying red (R), green (G), and blue (B) colors, an overcoat layer 23 for planarizing the upper substrate 20, and a common electrode 24 on the overcoat layer 23. At this time, an electric field inducing window 25 is formed in the common electrode 24 for differentiating the alignment direction of the liquid crystal molecules.
A fringe field is formed by the electric field inducing window 25 inside the common electrode 24. The liquid crystal molecules at both sides of the common electrode 24 are differently aligned in accordance with the fringe field. At this time, the auxiliary electrode 13 reinforces the fringe field in the periphery of the pixel electrode 15. Also, a liquid crystal layer of the liquid crystal molecules 31 is formed between the lower and upper substrates 10 and 20.
At this time, first and second alignment layers 16 and 26 are respectively formed on entire surfaces of the lower and upper substrates 10 and 20 for an initial alignment of the liquid crystal molecules 31. The first alignment layer 16 of the lower substrate 10 is deflected from the second alignment layer 26 of the upper substrate 20, whereby the liquid crystal molecules 31 adjacent to the first and second alignment layer 16 and 26 are aligned in accordance with the alignment direction of the respective first and second alignment layers.
An operation of the related art LCD device having the aforementioned structure will be explained as follows.
When a voltage is not applied, the longitudinal directions of the liquid crystal molecules are parallel with the lower and upper substrates 10 and 20, and the liquid crystal molecules are continuously twisted at 90 degrees. Thus, light moves along the longitudinal directions of the twisted liquid crystal molecules, whereby a screen becomes white. When a voltage is applied, as shown in FIG. 1B, the longitudinal directions of the liquid crystal molecules would be perpendicular to the lower and upper substrates 10 and 20, whereby a screen becomes black, except for the fringe field.
At this time, the fringe field is generated by the electric field inducing window 25 formed inside the common electrode 24, so that the liquid crystal molecules 31 are differently aligned according to the electric field inducing window 25, thereby compensating the viewing angle.
The auxiliary electrode 13 may be formed on the same layer as the data line 12 when forming the data line 12, or may be formed on the same layer as the gate line 11 when forming the gate line 11, whereby integration is improved. However, in this case, the auxiliary electrode 13 is formed to have a predetermined interval with the gate line 11 or the data line 12 to prevent electrical shorts, thereby decreasing a portion for forming the pixel electrode 15. Also, the auxiliary electrode 13 is made of a material that does not transmit light. As the auxiliary electrode 13 is overlapped with the pixel electrode 15, the aperture ratio becomes low, whereby brightness is reduced.
In the viewing angle aspect, the liquid crystal molecules 31 are slanted in region ‘A,’ where an electric field is generated between the auxiliary electrode 13 and the pixel electrode 15, so that a phase difference of light increases in ‘A’ region, thereby increasing light leakage, and decreasing a contrast ratio.
In case of the general TN mode LCD device in which the domain is not divided, the liquid crystal molecules 31 are at an 80 degree angle to the lower and upper substrates 10 and 20. Meanwhile, in case of that the TDTN mode LCD device in which the two domains are formed, the liquid crystal molecules 31 are slanted in the periphery of the pixel region due to the auxiliary electrode 13 inducing the electric field distortion. Accordingly, the phase difference of light passing through region ‘A’ increases, and the light leakage is generated in region ‘A’, whereby the contrast ratio decreases.
For preventing the light leakage, a black matrix layer may be additionally formed in region ‘A’. However, as the black matrix layer is formed, the aperture ratio decreases, thereby decreasing the brightness.